The present invention has as its object an optical device for concentrating the light emitted by an electroluminescent or light emitting diode, this device being integrated in the light emitting diode and formed within one of the layers of semiconductive material forming the diode. It is characterised in that at the surface of the diode, it comprises at least one semi-torus lens surrounding a central lens, the said torus forming a complete ring in the plane of the diode surface and being cut in the direction of its thickness. The invention equally relates to the light emitting diode with which this optical device is associated.
The invention is applicable to light emitting diodes in general, whether these are of the kind emitting through the top, meaning through the surface opposed to the substrate, which are better known as "top emitting", or of the "Burrus" type, that is to say emitting through a hole made in the substrate, the diode being inverted with respect to its substrate and welded to its base via deposited layers and no longer via its substrate, as in common practice. This is why the internal structure of the diode will not be given in particular in the description of the invention since it is not affected by the latter.
It is apt however to specify that the invention is applicable to the light emitting diodes produced from substances of the IIIrd to Vth groups and in particular alloys based on gallium and arsenic, such as GaAs and Ga.sub.x Al.sub.1-x As which are the most usual without thereby being restrictive.
The problem for which the invention provides a solution is that of the optical coupling between a light emitting diode and an optical fibre, for all the telecommunications, data processing and office automation systems making use of interfaces of the fibre type.
It is known that light emitting diodes have a very low external quantitative efficiency, commonly being ofthe order of 2%. The optical power plotted in a Lambertian emission graph is actually limited by total reflection within the semiconductive material which occurs at the interface between the semi-conductive element of the diode and the air.
The efficient solution of this power loss problem consists in installing an output lens at the surface of the diode. Adapting the refractive indices of the different media and the shape of the lens, renders it possible in this manner to recover a greater quantity of the light emitted by the active layer of the light emitting diode.
One of the problems, amongst others, is that of the dimensions of the optical device installed on the diode. The limitation of the thickness of the layers which may be deposited limits the radius of the lenses which may be cut in these layers and consequently the quantity of light recovered by each lens: a lens of large diameter recovers more light but requires a thick layer, whereas it is impossible to produce layers exceeding approximately 20 microns in thickness. To secure a more intense light beam at the extremity of the optical fibre, it has been proposed to associate several lenses side-by-side, corresponding to as many emissive portions within the body of the diode. Associating circular lenses does not provide an optimum however: a substantial fraction of the surface, in the form of curvilinear triangles inserted between the lenses, is unused in the upper plane of the diode, in which plane the lenses are contiguously situated.